This is a very sad accident for two-men who were just doing their every-day jobs so citizens can have sewage services. My thoughts and prayers go out to their families and friends.
I was curious about the environmental impact and researched in on the internet. It seems that the river flow rate and temperature will mitigate the oxygen depletion of the spill and it should rebound rather quickly. However; there is a flash effect to the system during an event like this.
Here is a website that simplifies this type of situation.
Hopefully, they will identify why this accident happened to help protect other workers that could be at risk.